An astute recently noted that buried inside of iChat are suggestions that both a tabbed messaging window, similar to Adium, and document sharing capabilities are in the works.
A icon labeled "people-ws.tif" appears to take the place of the microphone and video icons next to individual's names in the contact list. Further searching inside of iChat's Localizable.strings files indicates that "ws" likely stands for "Window sharing," which includes a note describing the Window sharing banner as an "Invitation to share a document with." The feature would enable one person to share a document with another and support both users editing the document simultaneously.
Buried inside of iChat are also images for Tabs, suggesting that Apple has already explored adding the popular feature.